1. Field of the Invention
The present invention relates to antennas installed on automobiles and used for receiving AM/FM bands and more particularly to a shortened mast antenna with compensating circuits.
2. Prior Art
When shortened mast antennas are used in automobiles for receiving AM/FM bands, a conspicuous sensitivity drop is likely to occur. Conventionally, it has been the practice to connect an AM broad-band amplifier and an FM broadband amplifier in parallel and insert these amplifiers between the antenna and a feeder line.
Specifically, when an AM/FM antenna is used in the FM frequency band, if such an antenna is shorter than the resonant state e.g., at a length of 50 cm (with a 6 mm diameter) which is approximately 1/2 the length which resonates at FM frequencies as shown in FIG. 6(2), then the antenna resistance Ra will become approximately 10 ohms (FIG. 6(1)). This is lower than the resistance in the resonant state (which is approximately 75 ohms) and results in an antenna reactance Xc of approximately -200 ohms (equivalent electrostatic capacitance: approximately 12 PF).
Automobile antennas usually have a telescopic structure so that the antenna is retracted inside the vehicle body when not used. As a result, the stray capacitance at the base of the antenna is generally 20 PF to 40 PF due to the mechanical structure involved. Because of this stray capacitance, the apparent antenna resistance becomes even lower.
If a commonly used coaxial feeder line (which has a characteristic impedance of 50 ohms to 200 ohms) is directly connected to such an antenna, the mismatch loss becomes larger and the band width becomes extremely narrow. Thus, it is impossible to get FM reception with good sensitivity. Conventionally, this problem has been solved by inserting broadband amplifiers between the antenna and the feeder line, as mentioned above.
If the AM/FM antenna is approximately 50 cm long so that it is used in the AM frequency band, such antenna length is extremely short compared to wavelengths in the AM frequency band. Accordingly, the antenna resistance Ra becomes virtually 0 ohms, and the antenna reactance Xc becomes -20 kilo-ohms to -50 kilo-ohms (equivalent electrostatic capacitance: approximately 7 PF), resulting in an extremely high-impedance antenna.
When an antenna and a radio receiver are connected by a coaxial feeder line, the feeder-line is shorter than the wavelength involved. Thus, in this case there is no need to consider impedance matching. However, there is a capacitance splitting loss arising from the antenna capacitance and the antenna stray capacitance plus feeder line electrostatic capacitance, resulting in a considerable drop in reception sensitivity.
Furthermore, in the case of a motor-driven antenna, the length of the feeder line reaches 4 to 5 m, and the electrostatic capacitance of the feeder line reaches 150 to 300 PF or greater. As a result, the splitting loss amounts to as much as -25 to -35 dB.
In view of the above, a low-capacitance cable with a high characteristics impedance is used in some cases in order to reduce the capacitance splitting loss. In such cases, however, the FM signal matching loss increases, and the FM reception sensitivity becomes poor.
Conventionally, therefore, a compromise between the above-described two situations has been adopted, and coaxial cables with a capacitance of 30 to 50 PF/m have been commonly used.
When strong electromagnetic waves are received in conventional devices mentioned above, the electromagnetic waves are amplified in the non-linear ranges of the broad-band amplifiers, so that amplitude distortion is generated, and the sound that is received is distorted.
Furthermore, when an attempt is made to receive other waves among strong electromagnetic waves, cross modulation distortion and intermodulation distortion are generated by the non-linear distortion of the broad-band amplifiers. As a result, not only is the received sound distorted, but reception may become impossible in some cases.
In addition, because of noise generated by the broad-band amplifiers, the practical reception sensitivity drops. In other words, the receiver input signal level required in order to achieve the prescribed S/N ratio, e.g., 20 dB in the case of AM broadcast waves and 30 dB in the case of FM broadcast waves, is increased.
Furthermore, since both AM and FM broad-band amplifiers are used, the overall cost of the antenna increases. If high-performance amplifiers with a high linearity are used to prevent such distortion of the received sound, the cost is increased even further.